


THE MATHEMATICS OF RELATIONSHIPS

by Hqarmystrings



Category: Haikyuu!!
Genre: Freak Duo give me and Tsukishima a headache, Gen, I read the light novel and went through it y'all, Lots of Maths Metaphors, M/M, Maths Metaphors, Pockets of attempted humour, Steins;Gate reference, There's no kissing but its pretty gay fellas, Tsukishima Kei is so done but also cares a lot
Language: English
Status: Completed
Published: 2020-09-21
Updated: 2020-09-21
Packaged: 2021-03-07 15:55:38
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,744
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26580250
Author URL: https://archiveofourown.org/users/Hqarmystrings/pseuds/Hqarmystrings
Summary: Maths was easy. 1+1=2, and you could solve for x by substituting it in the equation for y. Maths was easy. You applied formulas to solve the area of the shape.Maths was easy to comprehend.Kageyama and Hinata were not.In which Kei attempts to solve the conundrum of the Freak Duo through Mathematics and learns that relationships aren't that easy.
Relationships: Bokuto Koutarou & Kuroo Tetsurou & Tsukishima Kei, Hinata Shouyou & Kageyama Tobio & Tsukishima Kei & Yachi Hitoka & Yamaguchi Tadashi, Hinata Shouyou & Tsukishima Kei, Hinata Shouyou/Kageyama Tobio, Tsukishima Kei & Yachi Hitoka, Tsukishima Kei/Yamaguchi Tadashi
Comments: 52
Kudos: 265





	THE MATHEMATICS OF RELATIONSHIPS

**Author's Note:**

> Basically I read the light novel and got stressed over Kagehina and this is the result. Thank you to [Dani](https://archiveofourown.org/users/curryramyeon/pseuds/tinygumdrops) my absolute superstar and beta and friend without who I would not write a word. Thank you [Krys](https://archiveofourown.org/users/hqkrys/pseuds/hqkrys) for pulling through with a last minute beta and to [Yuzu](https://archiveofourown.org/users/mervousmer/pseuds/mervousmer) for convincing me that I wasn't completely insane for writing this. Here is a preview to the insanity on [twitter](https://twitter.com/Hqarmystrings/status/1307357467110645761?s=20). Thank you for reading this!

THE MATHEMATICS OF RELATIONSHIPS

_Two parallel lines never intersect._

Maths. Kei was good with Maths. It was simple, really - cold, hard logic. He could do that. Calm, calculated, logical—that’s what he was, the ordinary guy, who had blocked monsters like Ushijima and Hoshiumi. Maths was easy. 1+1=2, and you could solve for x by substituting it in the equation for y. Maths was easy. You applied formulas to solve the area of the shape. 

Maths was easy to comprehend.

Kageyama and Hinata were not.

_Two parallel lines never intersect._

Were Kageyama and Hinata parallel lines? Meant never to intersect but to run in the same direction, equidistant from each other? Kei pondered on the thought. 

But then they intersected a lot, an irritating number of times in fact. Their lives first intersected at Hinata’s first middle school match and again at the gym doors of Karasuno. The worst part was that Kei was now witness to this convergence, which seemed to include an annoying amount of competitive toothbrushing and squabbling over the last meat bun. The most frustrating part? This didn’t seem like the final intersection.

They weren’t two lines converging to meet at a final point. There was no final point for them. They were in constant movement, hurtling towards the place where monsters roamed freely. And yet they weren’t two lines intersecting at one point only and choosing to diverge after. They seemed destined to intersect again…

… Except two straight lines can only intersect at one point, mathematically speaking. 

Kei let out a “tch” at the logical inconsistency of the disaster duo, and decided they weren’t worth this much use of his precious brain cells.

\--------------------------------------------------------------------------------------------------------------------

“Human interaction can’t be solved by maths and you know that.” Tadashi is laughing far too hard at his dilemma and Kei feels a tad insulted. He scowls.

Tadashi smiles his soft, understanding, ‘captain’ smile at him. Kei hates it. That smile is for juniors who don’t know how to receive yet and for the new manager who is more of a nervous wreck than Yachi; it’s not meant for Tsukishima Kei, third-year, middle blocker, Cornerstone of Karasuno’s Defense.

“Okay, Tsukki, what are we then? If you’re going to use maths to solve human relationships, what is the ‘Equation of Us’?”

_The Equation Of Us_. Lame. Disgusting. Kei’s actually going to throw up. That’s so—

“We’re normal intersecting lines. We’ve intersected and one day we will diverge again.”

Tadashi’s smile saddens and Kei’s heart clenches as he hastily corrects his mistake. “But that day is probably the day one of us dies.”

The words are out involuntarily and Kei feels a little shocked at what he just said. Tadashi looks startled as well, which is fair. 

They’re in their third year. It’s a Friday. They’re not even sure if they’ll pass their Literature test next week or if they’ll make nationals this year or if Akiteru is cooking again tonight (he’ll stay at Tadashi’s house for dinner if he is).

But what Kei had said felt definite, a promise for eternity, which was _stupid_ , because they were still so young and the future was uncertain. So uncertain.

It was uncertain, and yet Kei had never felt more certain of something in his life. He knows that this is an intersection he doesn’t want to leave. Maybe the lines of fate are different than the lines of Mathematics. Maybe they aren’t lines at all but waves weaving in and out of each other creating a harmony of sound.The sound of _“What more do you need than pride?”_ and the squeak of the volleyball shoes at the gym. The sound of love. 

_Tch._ Kei’s beginning to hate how soft he’s getting nowadays, especially with Tadashi. 

Who cares about Maths. He’s planning to major in History anyway.

\-------------------------------------------------------------------------------

“OOH Steins;Gate!! Let’s watch that!!”

Kei winces at the volume at which Hinata yells his statement as he brandishes the remote. The dumbass is now at his house to technically study, but unfortunately they had just finished a test. Kei is in no mood to tutor anyone and he’s sure Hinata’s brain is fried to the point of no return.

“You’re too dumb for that show,” Kei says bluntly, resulting in an offended squawk.

“TAKE THAT BACK, STINGYSHIMA!”

Kei rolls his eyes and snatches the remote from him, and Hinata pouts. Kei frowns back. He hates it when Hinata pouts. The brat _knows_ that Kei hates it when he pouts.

Hinata is sulking now. “What’s the big deal? It’s just a regular anime.” 

Kei pinches the bridge of his nose and sighs. “It talks about alternate universes and branching timelines that eventually meet back at the timeline they originated from. Where’s the ordinary part?”

Hinata wrinkles his nose. “Sounds like a school test. Boring.”

Kei scowls at him. Steins;Gate is a masterpiece. Dumbass Hinata. God he hates how dumbass has become a part of his vocabulary because of the King. 

“Branching timelines that eventually meet back at the timeline that they originated from…” Hinata repeated slowly. He hums deep in thought. Kei is about to tell him to not burn an already fried brain when Hinata speaks up again. “Ah! Like me and Kageyama I guess!” Hinata grins brightly and then settles down on the sofa happily, as if he hadn’t just dropped the bomb of the century.

_What?_

“… Elaborate,” Kei says. “Now.”

Hinata looks at him surprised. Kei is aware that he sounds aggressive but he doesn’t care.

“Uhh… I guess it’s because we both like volleyball and we’re at Karasuno,” Hinata says. “But stupid Kageyama is going to the V.League now. And he didn’t even tell me. Asshole.” Hinata scowls at the sofa and picks at the material. It’s a good sofa. If it gets ruined, Kei is going to make him pay for a new cover.

“You didn’t tell him about Brazil,” Kei states matter-of-factly.

“THAT’S DIFFERENT, OKAY?” Hinata exclaims. “THAT’S… I don’t know--I just forgot to tell him.”

_These two_. Kei is getting a headache from all the Stupid.

“Have you considered that he didn’t tell you because of the same reason?”

Hinata stares at him moodily. 

Kei sighs. “I told you about Steins;Gate and the theory of time and you made some grand claim about how it reminded you of your relationship with Kageyama. How can you and the King be the same as that? I’m pretty sure Kageyama learnt how to _tell_ time just this year.” 

Hinata purses his lips to show he’s still sulking but Kei can see that the tiny hamster wheels in his brain are whirring. 

“Mmm… I feel like we come from the same place and we want to stand on the same stage but we won’t do it the same way.” 

Hinata now has That Look on his face. The same laser focus and alertness that only happens when it involves volleyball. Or Kageyama. Kei wonders if the terms ‘Volleyball’ and ‘Kageyama’ even exist separately in Hinata’s mind. They were probably a single entity at this point.

“Like two timelines diverging from the same point.” Hinata makes a vague motion with his hand. “Existing in two different timelines only to once again meet at the original timeline.”

“Only to once again diverge?” Kei asks and it’s meant to sound cryptic but the interest seeps through.

“Yeah, probably. He’s my partner but he’s also my rival,” Hinata says. “I want to crush him. I promised him that much and I hate breaking promises. I hate losing even more.” The Look is now in full force. Eyes sharp, aura calm yet deadly, body language similar to a predator. Almost feral. Like animals. 

Disgusting. 

“So what, you’ll never be partners again or something?” Kei tries to come across as flippant but he knows he’s failing. He has no idea why he’s this bothered. Hell, he _wants_ to see the duo go up against each other. And he’s definitely rooting for Hinata. It’s fine. Suga-san will root for Kageyama, probably. They all want to see that fateful rematch where a fully-loaded Hinata and a fully-sharpened Kageyama go up against each other. 

But he also doesn’t want to believe that the Spring Interhigh during their third year would be the final place to celebrate the glory of the Freak Duo. Kei wants the world to know how amazing his teammates are. He’ll take that information to his grave, though. 

“Huh? No? I’m not going to let him steal all the spotlight at the Olympics?” Hinata states matter-of-factly.

The Olympics. Of course. Emotion clouds Kei’s mind. He feels a sense of relief but tries not to show it. Hinata isn’t paying attention anyway; he’s deep in thought.

Kei thinks that Kageyama and Hinata aren’t too different from him and Tadashi.

Maybe they aren’t lines but waves, weaving in and out of each other, creating tight bonds and knots of companionship in the universe, that time and space couldn’t break. What do time and space have compared to the blood, sweat, and tears of team losses and victories, the sheer thrill of the game and the rivalry, the inexplicable wordless understanding they’d developed as friends…

He’s getting far too soft. Tadashi should be the only exception. He isn’t going to extend privileges to The Idiot Emperor and The Fire Demon.

“That whole timeline thing was very shoujo manga heroine of you.”

Hinata flushes and squawks, flustered.

“That’s rich coming from you, Mr. Equation of Us.”

……….

He’s going to kill Tadashi. 

\------------------------------------------------------------------------------------------------------------

“A square.”

Kei lets his head fall on the table with a resounding thump. This is what he got for calling Kuroo-san. He sends his apologies into the universe.

“I’m sorry. This was a mistake. Thank you for your time I’ll be leaving n—”

“I’m serious! Think about it. The four of you are all on different paths but interconnected with each other in an inexplicable way. A square.”

“A square is very explanatory though, Kuroo-san. There are four sides and four corners.”

“ _Stop_! Be more metaphorical, yeah? After all, didn’t you come up with ‘Equation of Us’?”

Tadashi should start his funeral preparations because Kei is definitely going to kill him.

“I didn’t!” Kei insists. “That was Tadashi!”

“Ahh, but you indulged him, didn’t ya, Tsukki?”

Kei considers just ending the call. He owes Kuroo-san nothing (he owes him everything).

“Can you tell me, which exact corner does the line connecting one corner to another corner come from?”

“….The corner from which I draw it from…”

“ I MEANT, IF YOU DIDN’T DRAW THIS SQUARE. TSUKKI, PLEASE COOPERATE.”

“…No, I guess. But Kuroo-san, this isn’t about the four of us. I’m asking about those two weirdos.”

“But you’re asking about those two because you care about the four of you as a whole, right?”

Kei really hates Kuroo-san’s creepy accuracy sometimes.

A familiar voice asks from the other end of the line, loud and booming, “Kuroo, how could you forget about Yacchan?”

Kei grimaces. Of course _Bokuto-san_ is there on the other end, too. Of course.

“Oh yeah. Okay same point but it’s a pentagon.”

“But then what about the third-years? What about the juniors? OHHHH, Kuroo, what about us?” Bokuto crows. “Tsukki, what about us?”

From the silence, Kei can tell that Kuroo-san is actually pondering upon the question. That never bodes well.

“We’re all a circle. Multiple points connected in an infinite loop. Maybe we lie at different points but ultimately we’re connected.”

It sounds profound.

“Yep. That’s what we are… A volleyball circle.”

It’s not profound.

“OHHH, Kuroo, a volleyball IS a circle.”

“Bokuto, sometimes your mind amazes me, bro.”

Kei cuts the call.

\------------------------------------------------------------------------------------------

“I thought those two reminded me of the second and minute hand of a clock,” Yachi states thoughtfully.

Kageyama ‘King of The Court’ Tobio is no longer the genius in his mind. Yachi ‘Interpreter Of Idiot Duo’ Hitoka has replaced him. Yachi continues her train of thought, unaware of the upgrade and reward ceremony going on in Kei’s brain.

“They’re always moving forwards, hurtling towards their goal,” Yachi says “I don’t think they intentionally meet or intentionally separate. But yet they’re inexplicably always somehow connected.” There’s a slight pause.” They’re really similar in some ways.” She sighs. “But they’re so weird I don’t get them.”

Kei can’t help it—he bursts out laughing, part shock, part joy, part relief.

Yachi stares at him, puzzled. “What’s so funny? I don’t get it.” 

“You’re right. The Freak Duo are really weird.” And he can’t help the fond smile that spreads across his face.

Yachi stares at him. “Kei, you’re getting soft.”

“Shut up.”

\---------------------------------------------------------------------------------------------

It’s the match they’ve been waiting for. MSBY Black Jackals versus Schweiden Adlers. Kageyama versus Hinata. The Freak Duo on opposite sides of the net.

A volleyball net has many intersecting lines _._

That’s what Kei vaguely thinks as he recounts the various events leading up to this fateful match. Hinata going to Brazil. Kageyama going to America.

_They’re inexplicably always somehow connected… Like the second and minute hand of a clock._

_Like two timelines diverging from the same point… Existing in two different branches only to once again meet at the original timeline._

Hinata runs parallel to the net and goes for a slide hit.

_Two parallel lines never intersect._

Kei sees Kageyama setting to Ushijima and Hoshiumi with pride and joy. He sees Atsumu bending over backwards to toss for Hinata. He sees Bokuto-san attempt to block Ushijima-san, both of them looking like they’re having the time of their lives.

_We’re all a circle. Multiple points connected in an infinite loop. Maybe we lie at different points but ultimately we’re connected._

He can spot former rivals seated afar. He wonders at how they all gathered here today to see two former teammates of just a regular high school volleyball club play against each other.

_That’s what we are… A volleyball circle._

As he sees Hinata spike with his left hand effortlessly, Kei thinks of the ordeals in Brazil he undertook to get here. To stand on the same stage as Kageyama.

_I feel like we come from the same place and we want to stand on the same stage, but we won’t do it the same way._

Kei feels the calm yet determined aura radiating from Hinata as he receives Ushijima’s spike. He sees the incredulity and frustration yet subtle admiration in Hoshiumi-san’s expression when Hinata blocks him.

_HINATA SHOYO, I’LL BE WAITING FOR YOU!_

_I hate breaking promises. I hate losing even more._

He sees Kageyama and Hinata jumping up against each other, battling on opposite sides of the net. He swears that he can almost see the Karasuno banner waving in the background, _‘Fly’,_ and it imprints in Kei’s mind. He can hear the crowd cheering and the blood pounding in his ears.

_Maybe the lines of fate are different from the lines of Mathematics. Maybe they aren’t lines at all, but sound waves weaving in and out of each other, creating a harmonic symphony._

Kei sees Hinata jump for the decoy, he sees Kageyama fall for it. He sees the pure happiness radiating from their faces after Hinata wins, and Kei feels the same because he knows that this isn’t the last for any of them.

_Maybe they aren’t lines but waves, weaving in and out of each other, creating tight bonds and knots of companionship in the universe, that time and space couldn’t break._

After the game, Kei doesn’t try to mask the fond smile that spreads across his face as he teases Kageyama for his loss. Yachi and Yamaguchi are laughing behind him and Hinata pats their arms and joins the good-natured ribbing. Kei’s smile only gets wider when his senpais join them, although he hides it this time with a snarky comment on his favorite Idiot Duo.

_What do time and space have compared to the blood, sweat, and tears of team losses and victories, the sheer thrill of the game and the rivalry, the inexplicable wordless understanding they’d developed?_

Not a thing.

\-------------------------------------------------------------------------

As Kei watches the rerun of the Olympic Match of Japan versus Argentina, he smiles at the fact that he had, once, played alongside those two. And as he sees the Freak Duo cause the world to tremble in fear and excitement with the insanity of their plays, he congratulates himself on the decision to take History and not Maths.

After all, History would explain the Duo better than Math ever could.

**Author's Note:**

> If you read this, I love you! You can find me on [twitter](https://twitter.com/Hqarmystrings).Thank you once again!


End file.
